When cooling a heated metal plate such as a steel plate having been rolled or reheated, it is the usual practice to apply a forced cooling by spraying a cooling liquid such as water to the upper and lower surfaces of the heated metal plate, with a view to improving the production efficiency or the material quality of the metal plate. The forced cooling is generally applied at present to a heated metal plate through an on-line cooling equipment comprising a plurality of rows of nozzles arranged at prescribed intervals in the width direction of the metal plate above and below the metal plate, by moving the metal plate horizontally in the longitudinal direction of the metal plate at a prescribed speed, and spraying a cooling liquid from the nozzles onto the upper and lower surfaces of the metal plate during moving through the cooling equipment. The cooling liquid is applied onto the metal plate in a state of mist, spray or laminar flow.
When forcedly cooling the heated metal plate by the above-mentioned method, there have been problems of the occurrence of strain in the metal plate and a non-uniform distribution of mechanical properties of the metal plate by cooling. This is attributable to the fact that, during forced cooling, since the end portions of the metal plate in the width direction as well as in the longitudinal direction are excessively cooled as compared with the remaining portion thereof, the metal plate is not uniformly cooled.
To avoid the above inconveniences, various studies were carried out to find a method for forcedly cooling a heated metal plate uniformly. Uniform cooling in the width direction of the metal plate has almost been made possible by, for example, controlling the amount of cooling liquid ejected along the width direction of the metal plate so as to provide a uniform cooling temperature in the width direction of the metal plate. However, an effective means is not as yet developed for uniform cooling of the metal plate in the longitudinal direction thereof.
Causes of the impossibility of uniform cooling of a heated metal plate when forcedly cooling the metal plate, are as follows. For example, when spraying a cooling liquid, by the above-mentioned method, to the upper and lower surfaces of the heated steel plate moving in a cooling equipment at a prescribed speed, the cooling liquid sprayed onto the lower surface of the steel plate drops down immediately after hitting the lower surface of the steel plate and poses no problem, whereas the cooling liquid sprayed onto the upper surface of the steel plate flows down in the directions shown by the arrows in FIG. 1 in the longitudinal direction of the steel plate 1 as well as in the width direction thereof from the edgesthereof after hitting the upper surface of the heated steel plate 1.
As a result, the cooling rate of the steel plate 1 near the edge thereof is higher than that of the remaining portion thereof. Therefore, the temperature of the steel plate 1 after the lapse of a certain period of time since the start of cooling becomes lower in the portion near the edge thereof than that in the remaining portion thereof. Thus the portion near the edge of the steel plate 1 is excessively cooled. FIG. 2 is a graph illustrating the temperature distribution of the steel plate in the longitudinal direction thereof in case that the steel plate 1 having in size a thickness of 20 mm, a width of 3,200 mm, and a length of 12,000 mm; and in temperature 810.degree. C. is forcedly cooled to attain a target temperature of 700.degree. C. at stoppage of forced cooling. According to FIG. 2, the middle portion of the steel plate 1 was cooled almost to the target temperature (700.degree. C.), whereas the leading and trailing end portions of the steel plate 1 were cooled to a temperature considerably lower than the above-mentioned target temperature.
FIG. 3 is a perspective view illustrating the shape of a steel plate 1 when the steel plate 1 cooled forcedly to the above-mentioned target temperature was further subjected to natural cooling to the ambient temperature. As shown in FIG. 3, in the portions (a) respectively having a length of about 750 mm from the leading and trailing ends of the steel plate 1 along the longitudinal direction thereof, a deformation (b) having a height of up to about 50 mm was occurred. FIG. 4 is a graph illustrating the results of a tension test carried out on test pieces sampled from a steel plate 1 subjected to a cooling treatment as mentioned above. As shown in FIG. 4, tensile strength of the steel plate 1 is remarkably higher in the portions (c) with a length of about 500 mm along the longitudinal direction of the steel plate from the leading and trailing ends thereof than the remaining portion of the steel plate.
As described above, when a heated steel plate is forcedly cooled by spraying a cooling liquid onto the upper and lower surfaces of the steel plate, the cooling rate becomes higher in the leading and trailing end portions of the steel plate than in the remaining portion of the steel plate, thus leading to an excessive cooling of the above-mentioned leading and trailing end portions. At the stoppage of forced cooling, therefore, the temperature of the above-mentioned leading and trailing end portions is far lower than the target temperature. As a result, deformation of the steel plate occurs during the natural cooling, together with non-uniform distribution of the mechanical properties of the steel plate.
When forcedly cooling a heated steel plate, the target cooling temperature should preferably be within a range of from 500.degree. to 650.degree. C. in view of actual operation. The reasons are as follows. When setting a target cooling temperature lower than 500.degree. C., for example, the ambient temperature, and then forcedly cooling the steel plate to the above ambient temperature, the strain occurring in the steel plate by forced cooling becomes increasingly larger, and it becomes more and more difficult to correct the strain. In the aspect of structure, the steel plate would contain much bainite, or would have a structure mixed with martensite, and this deteriorates the steel plate in toughness and makes it an unserviceable product. When the target cooling temperature is set at a temperature over 650.degree. C., on the other hand, forced cooling cannot refine the crystallization structure of the steel plate, and does not provide material quality improving effects on the steel plate. When forcedly cooling a heated steel plate, therefore, it is believed to be the optimum operating practice to set a target temperature of from 500.degree. to 650.degree. C. at stoppage of forced cooling and to apply natural cooling subsequently to the ambient temperature in view of the extent of strain caused by cooling, the limit of possible correction of the occurred strain, and the improving effects of material quality through cooling.
Under such circumstances, there is an increasing demand for a method for forcedly uniformly cooling a heated metal plate such as a steel plate, which has been hot-rolled or reheated, in the longitudinal direction thereof to a prescribed temperature without causing occurrences of strain and non-uniform distribution of mechanical properties of the metal plate, but such a method is not as yet proposed.